Pyrolysis is a thermal process in which hydrocarbons in a hydrocarbon stream are converted at an elevated temperature into lower molecular weight hydrocarbon molecules. Pyrolysis typically occurs at operating temperatures above 430° C. (800° F.). The term “pyrolysis” encompasses the decomposition of hydrocarbons in the presence of superheated water or steam (hydrous pyrolysis), for example, in the steam cracking of hydrocarbons.
Pyrolysis reactors have been used in refineries or chemical plants to produce different products from various hydrocarbon feeds for petrochemicals manufacture. For example, certain pyrolysis reactors are utilized to produce olefinic (e.g., alkenyl) materials. The feeds for this type of reactor typically range from ethane to vacuum gas oil, with heavier feeds giving higher yields of additional by-products, such as naphtha. As another example, pyrolysis reactors have also been used to produce alkynyl products, such as acetylene. The feeds for this type of reaction typically include methane.
As an example of the generation of alkynyl products, U.S. Pat. No. 2,319,679 describes one example of a pyrolysis system which may be used to produce acetylene containing product. The reactor in this system is a regenerative reactor or regenerative reverse-flow reactor in which heat is stored in refractory material for carrying out the pyrolysis of methane feed. During heating (e.g., regeneration), heat is supplied from the combustion of a fuel, with the hot combustion product flowing through the refractory material as a heat source. This heating is then stopped and methane is passed through the heated refractory material in an opposite direction of the flow of the combustion product. During pyrolysis, the methane is pyrolysed to form acetylene. The pyrolysis is then stopped and the flow of fuel is repeated to regenerate the reactor. The combustion of the fuel not only supplies heat for repetition of the pyrolysis, but results in burning of carbon materials that were deposited on the refractory walls during the pyrolysis.
As another example, U.S. Patent App. Pub. No. 2007/0191664 discloses a reverse-flow reactor in a pyrolysis system in which acetylene product is produced from a methane containing feed. The system includes a first and second reactor arranged in series to manage the flow of feed and product streams through the system. The reactors include multiple flow channels, such as channels in a honeycomb monolith arrangement, which provide increased efficiency and/or selectivity in producing acetylene product.
Because certain hydrocarbon feeds are more expensive than others, it is desirable to use lower cost, heavier hydrocarbon feeds in pyrolysis systems to produce olefins and acetylene products. However, the use of heavier feeds in pyrolysis systems tend to form higher amounts of coke and tar, which makes it more difficult to dispose of the heavier tar and coke by-products relative to lighter feed pyrolysis processes. This increase in coke and tar production is believed to result from the lower hydrogen content in the heavier feed.
Various other systems are able to utilize a quench fluid to quench the reaction and manage the production of coke with this fluid. As one example of a quench fluid, water has been used as a quench fluid in heavy oil pyrolysis processes. However, the water may result in the formation of emulsions that are difficult to manage in the process. Another example of is a quench fluid utilizes oil. Oil typically needs to be followed by a water quench to remove light condensable hydrocarbons from the stream, and the overall process is limited to a coke and tar production of up to about 30 weight percent (wt %).
Further, other regenerative processes may utilize a purging step to sweep the products out of the reactor. This step avoids contaminating products from one step with products from another step in the cyclical process. The purging step involves sweeping the purge fluid through the interior of the reactor to remove various vapor products. As an example, U.S. Pat. Nos. 2,629,753, 2,580,766 or 2,857,443 describe sweeping the reactor to remove the vapor products.
Other processes, such as steam cracking, may inject a fluid in equipment that is downstream of the steam cracking furnace when cracking heavy feeds. As an example, U.S. Patent App. Pub. No. 2008/0128330 describes a method of cleaning the downstream equipment from a furnace. In particular, the method involves the cleaning of a transfer line exchanger that is used to quench a steam cracking process. As the transfer line exchanger becomes coked and/or coated with a tar substance during the quenching step, a fluid is introduced into the transfer line exchanger to reduce the coke and tar build-up.
Because economics tend to further favor heavier feeds for pyrolysis processes, which result in increasing tar and coke formation, there is a need to develop improved pyrolysis processes that are more readily capable of handling a wider variety of hydrocarbon feeds than conventional pyrolysis processes for a regenerative reactor. There is also a desire of enhancing pyrolysis processes involving a regenerative reactor to increase the production of C2 unsaturates (e.g., acetylene) in the pyrolyzed product, while also reducing the problems associated with coke and tar build-up. Moreover, there is also a desire to prolong the operation of the process and remove contaminates.